The vote comes as China takes a tough line against any challenges to its sovereignty, with high-profile young candidate Agnes Chow barred from standing because her party promotes self-determination for the semi-autonomous city.
The atmosphere was tense early Sunday when a small group of people heckled Chow as well as leading pro-democracy activists Joshua Wong and Nathan Law near a polling station, according to an AFP reporter at the scene.
One of the men barged into Wong, who was one of the student leaders during mass demonstrations in 2014 calling for a greater democratic freedoms.
"When there is a restriction on freedom of speech and we face more suppression on civil disobedience and protest in the streets, it proves that it's more necessary for us to vote," Wong told reporters.
Beijing has become increasingly incensed at the emergence of activists advocating independence and sees calls for self-determination as part of a dangerous splittist push.
The by-election was triggered after Beijing forced the disqualification of six rebel lawmakers who had swept to victory in citywide elections in 2016.
Some were former protest leaders, others openly advocated independence. All were ousted from their posts for inserting protests into their oaths of office.
Four of the six vacant seats are being contested Sunday.
"The election is not just about selecting me as a candidate, it is also about voting for justice," said Au Nok-hin, who stepped in to contest the Hong Kong Island seat after Agnes Chow was disallowed.
The seat was originally held by Law, also a 2014 protest leader, who was among the six thrown out of office.
But pro-establishment politician Judy Chan, standing against Au, cast the opposition as provoking "violence and resistance".
"The by-election is a chance for the silent majority, who are tired of a politicised Hong Kong, who detest those who humiliate the country, to come out and tell those politicians that Hong Kong has no room for them," Chan told AFP.
Democracy camp struggling
Many of the first people to cast ballots at a polling station AFP visited early Sunday were more elderly and supportive of the city's pro-Beijing establishment.
"I'm not resistant to democracy and freedom, but I can't accept the independence of Hong Kong," voter Chan Chik-sing, 60, told AFP.
"If the government's policies were really that good and supported by citizens, the public's satisfaction level for the government wouldn't keep deteriorating," said voter Lilian Leung, who was in her late 30s.
The six lawmakers were retrospectively barred from office by Hong Kong's high court after Beijing issued a special "interpretation" of the city's mini-constitution stipulating legislators had to take their oath "solemnly and sincerely" or face being banned.
Pro-independence lawmakers had inserted expletives and waved "Hong Kong is not China" banners during their swearing in. Others added phrases supporting the democracy movement.
The pro-democracy camp has come up against increasing pressure since the failure of the Umbrella Movement to win political reform, with some leading activists jailed on protest-related charges.
Political analyst Dixon Sing says losing any one of the four by-election seats would be a further blow.
"It would only add to the disappointment and the loss of faith," he told AFP.
But Sing added that even an across-the-board win would be countered by a system fundamentally weighted towards Beijing.
Only half the legislature is elected, with the rest selected by traditionally pro-establishment interest groups.
Of 70 seats, the democracy camp currently holds 24, only just clinging on to the one-third needed to veto important bills.
It has also been curbed by new rules against filibustering, long a favoured tactic.
Nevertheless, veteran democrats are urging residents to go out and vote.
"It is not just a by-election," said pro-democracy lawmaker Claudia Mo.
"It's a fight between good and evil."
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)
Follow NDTV for latest election news and live coverage of assembly elections 2019 in Maharashtra and Haryana.
Subscribe to our YouTube channel, like us on Facebook or follow us on Twitter and Instagram for latest news and live news updates.